I am thinking about buying a 2004 TJ with a 6" skyjacker kit already installed running 35" tires. The owner does not know whether it is a long arm or short arm kit. I do know that it doesn't have a sye kit on it. I want a long arm suspension on my jeep. Are there any kits that i can use to simply upgrade the suspension to a long arm i.e Rusty's or RC or RE. Also what other problems should i expect from running 35's on a short arm suspension?
thanks in advance.

Well with that much lift you are going to have serious drive line vibes, 4" is plenty for 35s, yes you can buy a conversion to long arm, you will also need to buy a SYE, and CV drive shaft. Short arms and 6" of lift will cause all kind of problems, I wouldn't buy it if I were you try to find something else with less modification. Also depending on the axles you may need to replace or upgrade them as well.